1. Field of the Invention
This invention relates to compressed air energy storage systems for use with electric utility operations supplying electricity to a community of consumers. More particularly, it relates to a closed system for energy storage utilizing energy produced by one or more electric utilities.
2. Prior Art
The increased urgency of energy conservation has prompted both private and government studies for improving efficiency of current energy systems, as well as developing and commercializing new energy sources. As part of these efforts to improve efficiency in energy production, specific studies have been directed toward the storage of energy produced by electric utility systems during low demand periods, with subsequent regeneration and distribution during pack demand periods for the communities being supplied.
In accordance with this approach, the conventional integrated generating facilities of electric utilities can be designed with a maximum power output which is lower than the peak power demand, based on projected system needs. The difference between this peak demand and the conventional design output of the utility is developed from stored energy which is converted to electrical energy and added to the maximum conventional utility output to reach the total peak demand energy requirement.
This regenerated energy is produced from stored energy produced from the utility operation during low demand periods, when the utility output capacity exceeds the actual demand experienced. Such low demand periods would include nighttime and weekends. Conceptually, this process is represented by a sinusoidal curve (FIG. 1) in which line A--A represents the maximum conventional (base load and integrated peaking) capacity of a given utility or electric plant, and the sinusoidal curve 10 represents consumer demand for energy. During trough periods 11 when consumer demand 10 is below the utility maximum capacity represented by line A--A, excess energy is produced and stored for retrieval at peak demand periods 12.
Several significant conservation factors occur in such a system. First, the capital outlay for construction and operation of the electric utility plant is significantly reduced when the maximum energy output conventional capacity for production of utility is reduced from line A'--A' to line A--A. In addition to this lower capital requirement, a second economy occurs with the more efficient use of equipment involved. For example, instead of the utility plant operating at only 1/3 capacity during low demand periods 11, the utility can operate at the more economical energy production rate occurring at or near full capacity. This factor is particularly significant when the low cost of abundant fuel such as coal is compared with the high cost of equipment and operation associated with a typical utility.
These factors, along with other economical advantages relating to cost efficiencies of production, have encouraged the U.S. Government to assist in the research effort to improve the efficiency of electric utility operations. One such system has been denominated the compressed air energy storage (CAES) system which stores energy in the form of compressed air produced during low demand periods and retrieves such stored energy peak demand periods for distribution to the consumer. The CAES system typically involves the use of large, underground caverns as the compressed air storage site. As typified by the structure in FIG. 2, air is compressed by motor-driven compressors (left of figure) and is stored in the underground reservior. When supplemental power is required, the stored air together with added hydrocarbon fuel is expanded through a combustion turbine to thereby drive an electric generator.
A primary limitation for the use of CAES systems involves the requirement for very large, underground caverns. Although abandoned mines are extremely suitable, their geographical location may not be in the area of the particular electric utility. Considerable interest is also developing in salt mining technology for possible formation of underground caverns in salt domes or beds throughout the country.
A more detailed discussion of the CAES system, along with additional refinements and engineering improvements are contained in a publication of the U.S. Energy Research and Development Administration, ERDA 76--76, "Economic and Technical Feasibility Study of Compressed Air Storage" (March, 1976), and a followup report entitled, "Conceptual Design for a Pilot/Demonstration Compressed Air Storage Facility Employing a Solution-mined Salt Cavern" EPRI EM-391 (June, 1977). These publications describe actual systems designed for location at McIntosh Salt Dome, Alabama and Huntorf, Germany. Although various improvements directed toward optimizing the CAES system have been reported in these publications, the basic scheme of operation represented by FIG. 2 remains unchanged. Furthermore, the current state of the art of such energy storage systems continues to perpetuate significant disadvantages including (1) consumption of fluid hydrocarbons, resulting in accelerated depletion of natural gas and petroleum reserves, (2) operation at higher costs than base load coal fired or nuclear units, (3) limitation as to selection of working fluid within the CAES system, and (4) pollution of environment by combustion of hydrocarbon fuels as opposed to cleaner fuel systems.